Conventional fuel systems for vehicles with internal combustion engines can include a canister that accumulates fuel vapor from a headspace of a fuel tank. If there is a leak in the fuel tank, the canister, or any other component of the fuel system, fuel vapor could escape through the leak and be released into the atmosphere instead of being accumulated in the canister. Various government regulatory agencies, e.g., the U.S. Environmental Protection Agency and the Air Resources Board of the California Environmental Protection Agency, have promulgated standards related to limiting fuel vapor releases into the atmosphere. Thus, it is believed that there is a need to avoid releasing fuel vapors into the atmosphere, and to provide an apparatus and a method for performing a leak diagnostic, so as to comply with these standards.
In such conventional fuel systems, excess fuel vapor can accumulate immediately after engine shutdown, thereby creating a positive pressure in the fuel vapor pressure management system. Excess negative pressure in closed fuel systems can occur under some operating and atmospheric conditions, thereby causing stress on components of these fuel systems. Thus, it is believed that there is a need to vent, or “blow-off,” the positive pressure, and to vent, or “relieve,” the excess negative pressure. Similarly, it is also believed to be desirable to relieve excess positive pressure that can occur during tank refueling. Thus, it is believed that there is a need to allow air, but not fuel vapor, to exit the tank at high flow rates during tank refueling. This is commonly referred to as onboard refueling vapor recovery (ORVR).
A disadvantage of a conventional natural or passive vacuum evaporative leak detection system is that the testing pass/fail threshold is too low. That is to say, the leakage required to fail an evaporative leak detection test is relatively small. It is desirable for a test to fail when leakage is just below the required limit set by the various government regulatory agencies. This would maximize the opportunity to locate, and then repair, a system leak. This is particularly difficult in compact and sub-compact automobiles, which typically have small fuel tanks and tightly packaged underbody components. As a result, the small surface area and poor convection properties thermally isolate the evaporative leak detection systems and slow heat transfer. It is believed that, for sealed fuel systems, the rate of heat transfer is not a variable that contributes to the generation of pressure or vacuum. However, when a leak is introduced to the fuel system, the rate of heat transfer becomes a predominate variable.